Magnesium-lithium-yttrium alloys



3,139,442 MAGNESlUM-LTTl-ITUM-YTTRIUM ALLQYS Paul D. Frost and Thomas G.lsyrer, Columbus, Ohio, assignors, by mesne assignments, to the UnitedStates of America as represented by the Secretary of the Army NoDrawing. Filed May 27, 1963, Ser. No. 283,643 4 Claims. (Cl. 75122.7)

This invention relates to magnesium base alloys and more particularly tomagnesium base alloys containing lithium and yttrium. Further, itconcerns the beta phase of magnesium-lithium alloys employing yttrium.

Recent developments, particularly in the aircraft industry whereinstressed parts are subjected to elevated temperatures, have made itdesirable to provide metals of light wei ht which have good stabilityand hardness at these elevated temperatures. Although the magnesium basealloys are among the lightest of the structural alloys, they have notmet with wide acceptance in applications where stressed parts areexposed to elevated temperatures. Previous work has been done withmagnesium-lithium alloys in an effort to increase hardness of the alloyat elevated temperatures without loss of the desirable properties suchas lightness and ductility. These desired results are accomplished inthis invention wherein the magnesium-lithium alloy is combined withyttrium to form an alloy with good stability and hardness at elevatedtemperatures.

Therefore it is an object of this invention to provide an improvedmagnesium base alloy.

it is a further object of this invention to provide a magnesium basealloy with improved hardness at elevated temperatures.

it is yet another object of the instant invention to provide a betaphase magnesium-lithium alloy with good stability at elevatedtemperatures.

These and other objects of this invention will become evident uponreading the more detailed description hereinbelow.

In general the objects of this invention are accomplished by alloyingmagnesium and lithium in the ratio by weight of 7.5 magnesium to llithium, and adding amounts of yttrium not to exceed 6 percent of thetotal weight of the alloy. The ratio of 7.5 to 1 is necessary to insurethe formation of the beta phase of the alloy wherein the lithium contentcannot exceed 12 percent of the total weight of the alloy.

The order of addition of the metals to the magnesium is immaterial. Themagnesium and lithium may be melted together and the yttrium added orthe yttrium may be added to the molten magnesium prior to the additionof the lithium.

In accordance with the invention 10 to 12 percent by weight lithium and4 to 6 percent by weight yttrium are alloyed with magnesium to make a100 percent alloy. The magnesium and lithium are melted in an open topcrucible with a flux protective cover. An argon mantle can be usedinstead of the flux protective cover. Any suitable flux known in the artcan be used in this operation. The crucible is equipped with a stirringrod having a one inch flat disk fastened to the end. The yttrium ispuddled into the solution using this stirring rod to insure completestirring of the melt and better control over the addition of theyttrium. After alloying has been effected the melt is allowed to remainundisturbed for a time to permit separation of the flux as well asunalloyed metal, if any, thereby obtaining a clean melt of the alloy.This period is not required if the argon mantle is used. The settledalloy then is transferred into a suitable mold to solidifyv Smallamounts of a metal selected from the group consisting of thorium andaluminum may be employed in dfldhfldz Patented June 15, 1965 the betaphase magnesium-lithium-yttrium alloy. When used in amounts up to 5percent these metals help to improve the stability of the alloy atelevated temperatures. These metals are integrated into the alloy in thesame manner as described previously.

This invention is further illustrated by the following examples Withoutbeing limited thereto.

Example 1 A mixture of 83.8 grams of magnesium and 11.2 grams of lithiumare melted in an open top crucible with a flux protective cover. Asuitable flux is one composed of 57 parts of KCl, 28 parts of CaCl 12.5parts of BaCl and 2.5 parts of CaF Five grams of yttrium are added tothe mixture using a stirring rod with a one inch diameter flat diskfastened to the end. This allows for complete stirring of the melt andbetter control over the addition of the yttrium. After alloying has beenefiected, the melt is allowed to remain undisturbed for a time to permitseparation of flux as well as unalloyed metal, if any, thereby renderinga clean melt of the alloy. The settled alloy is then transferred to asuitable mold, such as a sand or a metal mold, to solidify. The ingots(1%" x 1%" x 3") are subsequently hot forged at a temperature of 500 F.to a y -inch thick plate.

Example 2 A mixture of 84.7 grams of magnesium, 11.3 grams of lithium, 2grams of yttrium, and 2. grams of thorium are alloyed in the same manneras Example 1. The thorium is added in the same manner as the yttrium.The alloy is forged as in Example 1.

Example 3 A mixture of 83 grams of magnesium, 11 grams of lithium, 2grams of yttrium, and 4 grams of aluminum are alloyed as in Example 2.The alloy is forged as in Example 1 but at a temperature of 550 F.

The following table illustrates the hardess of each of the alloys in theabove examples. An alloy of 12 percent lithium and 88 percent magnesiumis used as a reference.

Diamond Pyramid Forg- Hardness, kgjmrn. Nominal Alloy Composition, ing

Wt. Percent Telmp F. F. F.

178 Mg-12 Li 450 33.2 4.2 0.9 196 Mgll.2 Li5Y 500 29.6 25.4 15.4 9.1Mg-11.3L12Y Tl 599 45.9 19.5 9.6 4.9 197 Mg-11Li-2Y4Al 550 89.0 31.717.8 9.1

It is to be understood that the examples are for purposes ofillustration and that the invention is limited only by the appendedclaims.

We claim:

1. A magnesium base alloy consisting essentially of about 10 to 12percent by weight lithium and about 4 to 6 percent by weight yttrium,the balance being magesium.

2. A magnesium base alloy consisting of about 10 to 12 percent by weightlithium, about 1 to 3 percent by weight yttrium, and about 1 to 5percent by weight of a metal selected from the group consisting ofthorium and aluminum, the balance being magnesium.

3 4 3. The alloy of claim 2 wherein said metal is thqriurn. OTHERREFERENCES 4. The alloy of clalm 2 whereln sald metal 1s alumlnum.smith: College Chemistry, Sixth Edition Appletom References Cited by theExaminer Century Co., Inc., New York and London, pp. 569-570.

FOREIGN PATENTS 679' 747 9/52 G t B t 75 168 5 CARL D. QUARFORTH;Primary Examiner.

, rea r1 am 683,812 12/52 Great Britain 75-168 REUBEN EPSTEIN Examme

2. A MAGNESIUM BASE ALLOY CONSISTING OF ABOUT 10 TO 12 PERCENT BY WEIGHTLITHIUM, ABOUT 1 TO 3 PERCENT BY WEIGHT YTTRIUM, AND ABOUT 1 TO 5PERCENT BY WEIGHT OF A METAL SELECTED FROM THE GROUP CONSISTING OFTHORIUM AND ALUMINUM, THE BALANCE BEING MAGNESIUM.